Method and Apparatus for Cooling and/or Deep-Freezing Products, Especially Food Products, Implementing the Injection of Two Cryogenic Liquids

ABSTRACT

The invention relates to a method for cooling materials, in particular food, in a cooling apparatus, the cooling being completely or partially carried out by contacting the materials with a cryogenic liquid. Said method is characterized in that the contacting is carried out by means of the injection of two cryogenic liquids, nitrogen and CO 2 . The liquid nitrogen and the liquid CO 2  are injected either separately, into at least two injection points of the apparatus, or by mixing both cryogenic fluids at the injection point. The invention is of use if the apparatus being utilized is a mixing, batch mixing, or grinding chamber, with the injection being carried out into the body of the material in the bottom portion of the chamber.

The present invention relates to the field of processes for chillingand/or deep-freezing the contents of a chamber using a cryogenic liquid.It relates, in particular, to the chilling of food products in devicesof the following type: tunnels, mixers, blenders, grinders or doughmixers, churns, drums (“tumblers” in the literature), etc., it beingpossible for the contents of the device to then be solid or pasty, as isthe case for meat, or else liquid.

Although, in what follows, the case of food products is moreparticularly explained, the invention should not in any way berestricted to their case, it relates much more generally to many otherproducts, and especially chemical products, biological products, stemcells, etc. that undergo such cryogenic chilling operations.

In what follows, the case of grinder blenders is described in greaterdetail in order better to set down the ideas of the problems that arefaced.

In such applications of the use of cryogenic liquids for chilling foodproducts in grinder blenders, the use of CO₂ is favored, for itscapacity to transfer a lot of refrigeration at the change of state.

By considering the example of meat mixers, it is known that there is awealth of literature relating to the use of liquid CO₂, veryparticularly by injection into the bottom part of the blender, in orderto improve the heat exchange conditions between the cryogenic liquid andthe meat. Reference will be made, for example, to documents U.S. Pat.No. 4,476,686 and EP 744 578.

The application to the field of meat is indeed quite massive andemblematic (the products in question are very varied, ground beef (beef,veal), minced meat, ground pork (sausages, etc.), ground poultry (cordonbleu, nuggets, etc.)), of a field where the temperature control in theblender must be very effective:

-   -   it is necessary to compensate for the mechanical heating linked        to the mixing and mincing;    -   it is desirable to obtain a texture that is compatible for the        subsequent forming.

But it should be noted that for several reasons the demand of thisindustrial sector for the temperature control of grinder blenders isoriented toward the use of liquid nitrogen. Yet it is known that, forliquid nitrogen, the utilization of refrigeration at the change of stateis half that for CO₂.

In what follows, the main characteristics of the processes for the highor low injection of CO₂ or of nitrogen in such blenders are thensummarized.

High CO₂ Injection:

-   -   Only the solid phase of the CO₂ is utilized but the        refrigeration efficiency is advantageous at 64 kcal/kg at 20        bar.    -   The technique requires an optimum charging level and needs the        number of discharge horns (injectors) to be adapted so as to        deposit the carbon dioxide snow on the whole of the surface of        the meat without creating piles of snow, but it must be        acknowledged that the high injection implementation is easy.    -   It is characterized by a great ease of mixing of the solid        meat/solid snow phases, the carbon dioxide snow is generated at        the heart of the product.    -   The technique still has the risk of extracting the carbon        dioxide snow, it is therefore preferable to favor the overflow        extraction method.    -   This technique is conventionally limited to small mixers and        small production volumes (typically less than 100 tonnes per        year).    -   CO₂ furthermore has a bacteriostatic effect, it limits the        growth of microorganisms.

High Nitrogen Injection:

-   -   As indicated above, this technique is limited by a low        refrigeration efficiency, in the vicinity of 36 kcal/kg at 1.5        bar.    -   It is also characterized by the fact that it has risks of cold        spots and therefore by a difficult distribution.    -   The high nitrogen injection therefore makes it necessary to        monitor and control a low injection pressure.    -   For all of these reasons, it should be noted that the high        nitrogen injection is not used very much.

Low CO₂ and nitrogen injection: it makes it possible to utilize thelatent heat of the change of state of the cryogenic fluids and also aportion of the specific heat of the gases. This utilization of the gasesdepends on the contact time with the product.

Although in high injection the liquid nitrogen had a very largerefrigeration efficiency handicap compared to CO₂, in low injection, therefrigeration efficiency of the nitrogen approaches that of CO₂ (thecontact time between the gas and the product makes it possible toutilize the gases). Nitrogen furthermore has the advantage of offering asolubility in fats and water that is much lower than CO₂.

The fluid consumption observed is around 20% larger in low nitrogeninjection compared to low CO₂ injection. Within this entire context, itis understood, and this is one of the objectives of the presentinvention, that it would be advantageous to be able to have a novelprocess for chilling products in such devices, and especially inblenders with low injection of fluid, a process that enables a betterutilization of the gases and especially that makes it possible toutilize the portion of the gases which is not currently utilized inexisting processes.

As will be seen in greater detail in what follows, the process accordingto the invention, for chilling products in a chilling device, using acryogenic liquid brought into contact with the products, is noteworthyin that a better use of the gases will be obtained owing to theinjection not of a single fluid but of two fluids—liquid nitrogen andliquid CO₂— and through exchanges of refrigeration between the CO₂ andthe nitrogen, it being possible for the liquid nitrogen and the liquidCO₂ to be injected according to the invention either separately at atleast two injection points of the device, or by carrying out the mixingat the injection point(s) itself (themselves).

By once more setting down the ideas in the case of the example of thelow injection in a blender, the process according to the invention, forchilling a mass of product contained in a chamber (of mixer, blender,grinder, etc. type), using a cryogenic liquid injected within the massof material in the bottom part of the chamber, is noteworthy in that abetter utilization of the gases will be obtained owing to the injectionof two fluids—nitrogen and CO₂— and not a single fluid, and throughexchanges of refrigeration between the CO₂ and the nitrogen.

The present invention thus relates to a process for chilling products,especially food products, in a chilling device, the device used being achamber of mixer, blender, or else grinder or dough mixer type, whichmay contain a mass of product to be chilled, using a cryogenic liquidinjected within the mass of material in the bottom part of the chamber,being characterized in that two cryogenic liquids, nitrogen and CO₂, areinjected within the mass of material in the bottom part of the chamber,the liquid nitrogen and the liquid CO₂ being injected either separatelyat at least two injection points of the device, or by producing an insitu mixture at at least one injection point.

It will be noted that among the wealth of literature relating to theinjection of a cryogenic liquid into a cryogenic chilling device,document EP 1 887 296 is found, which relates to the production ofcryogenic mixtures for supplying product chilling devices. This documentconsiders that it is not satisfactory to inject different fluids viaseparate routes, it recommends producing a mixture upstream of thechilling chamber and injecting this pre-made mixture, it then mixescryogens (gaseous and/or liquid and/or solid cryogens) in a veryconventional manner via the use of an upstream mixing chamber, etc.

It will be shown below, especially via comparative examples but also viaextremely efficient methods of producing mixtures at the connectionpoint on a device of blender type, that the analysis that this documentmade is erroneous in the case of blenders with low injection: theseparate injections of liquid nitrogen and liquid CO₂ on the one hand,and on the other hand the low injection of mixtures of liquid nitrogenand liquid CO₂ by producing the mixture at the injection point(s) on theblender give remarkable results, and effectively enable a betterutilization of the gases through refrigeration exchanges between the CO₂and the nitrogen.

According to one of the implementations of the invention, there isadditionally, in the top part of the chamber, a system of forcedconvection that makes it possible to recycle and use the refrigeratingpower of the cold gases resulting from the low injection of thecryogenic liquids.

This system of forced convection may be formed by using, for example,fans or else by using a turbine, for example, by way of illustration,fans of 0.38 kW type equipped with 5 blades inclined at 45°.

According to one of the implementation methods of the invention, the twocryogenic liquids, nitrogen and CO₂, are injected at at least oneinjection point of the device, by producing the mixture at the injectionpoint, the injector used making it possible to carry out an exchange ofrefrigeration between the two liquids at the point of injection.

According to one of the forms of such an implementation where the mixingis carried out at the injection point, the injector used is aconcentric, twin-tube injector, the liquid CO₂ preferably passingthrough the outer tube (the “coldest” temperature passing through theinside, the “highest” temperature passing through the outside, incontact with ambient temperature).

According to another of the forms of such an implementation where themixing is carried out at the injection point, the injector used is aconcentric, three-tube injector, preferably using the fluids in thefollowing manner:

-   -   liquid nitrogen passes through the inner tube;    -   liquid CO₂ passes into the annular space between the first tube        and the second tube which is concentric thereto;    -   liquid nitrogen passes into the annular space between the third,        outermost tube and the second tube which is concentric thereto.

As will be better illustrated below, the experiments carried out by theapplicant clearly demonstrate the positive contribution of an injectionof two cryogenic liquids instead of one, to several parameters andperformances governing such a chilling process.

Without being in any way limited by the explanations that the applicantputs forward below, it may be considered that the following phenomenatake place, very advantageously.

Two cryogenic liquids are injected into the device, and exchanges ofrefrigeration between the CO₂ and the nitrogen take place, exchangeswhich are extremely valuable as will be seen.

The sub-cooling of the snow formed is especially witnessed (it is knownthat by being at atmospheric pressure the liquid CO₂ injected changes tothe form of snow and gas), which sub-cooling increases the capacity fortransferring refrigeration to the product.

Furthermore, by considering such situations of low injections in mixers,blenders, etc., witnessed here in all likelihood is the fact that theliquid nitrogen by releasing its kcal into the product, generates gashaving a very low temperature in the top of the equipment, and that thenthe gaseous CO₂, rising toward the top of the equipment, solidifies whenin contact with the very cold nitrogen present in this gas overhead,which snow may again return to be in contact with the product andtransfer its refrigeration to this product (in a way as in a “highinjection” type process).

Moreover, it is understood that then the presence of a forced convectionadded to the top part of such a blender (for example, via the presenceof a fan) may also increase these transfers.

The invention could furthermore adopt one or more of the followingtechnical characteristics:

-   -   the two fluids are injected separately at at least two injection        points on the device, and the following three steps for        controlling the process are implemented:        -   i) the two fluids are injected continuously until a setpoint            temperature is obtained in the mass of product treated;        -   j) the injection is stopped for a given stop time (for            example a few tens of seconds, for example 30 seconds);        -   k) the temperature in the mass treated is measured: if the            measured temperature is substantially equal to the desired            setpoint temperature the blender is changed to shutdown            mode, whereas if the measured temperature is greater than            the setpoint temperature, the injection is restarted until            the setpoint temperature is obtained.

The expression “shutdown mode” is understood to mean the cessation ofblending (and of the injection), the operator can then empty the blenderwhen he considers it appropriate.

This embodiment is very particularly advantageous for treating batchesof materials that are very different (quantity, quality, especially interms of fat content, etc.) and especially for adapting to the fact thatthe present invention enables, as will be seen further on, significantreductions in treatment times and therefore a considerably improvedproductivity, the fact of thus reducing the cycle times having to becarried out without at any moment taking the risk of generatingdifferent temperatures according to the volume of product treated.

-   -   According to one implementation method of the invention, in        order to avoid carrying carbon dioxide snow into the extraction        system of the chilling device, it is necessary to promote the        absorption of the snow generated in the top part of the chamber        by the product, it is then proposed to size the extraction        system so as not to promote gas velocities that are capable of        conveying snow. Although the desired gas velocities will be        different depending on the configuration of the chilling device,        the use of extractions referred to as “overflow extractions”        will be preferred according to the invention.

It is recalled that those skilled in the art of chilling or freezingequipment know the principle of these extractions referred to as“overflow extractions” (as illustrated highly schematically in FIG. 7below), where either a space is left between the equipment and theextraction line, or the extraction line itself is cut at one location.

The advantages of such a configuration within the context of the presentinvention are in particular the following:

-   -   the “break” enables the discharging of the condensates resulting        from the discharging of such cold gases, while limiting the risk        of these condensates returning into the equipment;    -   the “break” makes it possible to limit the discharge velocities        and especially to avoid discharge overvelocities and therefore        the velocities that would make it possible to discharge not only        gas but also snow which forms in the top of the blender        (according to the refrigeration exchange mechanism described        above as obtained owing to the present invention).    -   According to one preferred method of implementation of the        invention, use is made, on the liquid nitrogen reservoir        supplying the device, of a pressure regulation of the liquid        phase (regulation of the bottom of the tank), in order to        promote conditions where the amount of liquid nitrogen reaching        the device is regulated, irrespective of the fill level of the        nitrogen reservoir, therefore the height of liquid (“liquid        column”).

Reference could be made, for embodiment examples of this regulation ofthe bottom of the tank, to document WO 2004/005791 A2, for example byacting on the pressure of the gas at the top of the reservoir, forexample by vaporizing liquid withdrawn from the bottom of the reservoirin order to form gas sent to the top of said reservoir.

Other features and advantages of the present invention will thus becomemore clearly apparent from the following description, given by way ofillustration but implying no limitation, in conjunction with theappended drawings in which:

FIG. 1 is a schematic representation of a conventional mixer from theprior art (for example a meat mixer) having two troughs, employing, oneach side of the mixer, a series of liquid nitrogen injection nozzles inthe bottom part of the mixer;

FIG. 2 illustrates one method of implementation of the invention in ablender having one trough, using two separate injections of the twofluids, carried out on the same side of the trough;

FIG. 3 provides a partial view of the top part (cover) of a mixingchamber in accordance with one of the implementations of the invention,the top part being provided with a forced convection system formed bytwo fans having five blades inclined at 45°;

FIG. 4 provides a summary table of tests of implementation of theinvention and comparative tests;

FIG. 5 provides an example of a twin-tube injector that makes itpossible to carry out the mixing at the injection point;

FIG. 6 provides an example of a three-tube injector that makes itpossible to carry out the mixing at the injection point; and

FIG. 7 provides a very partial diagram illustrating an overflowextraction structure, in connection with the top of a blender.

FIG. 1 shows the lower part of a conventional mixer from the prior art(for example a meat mixer), having two troughs 2 and 3, for which aseries of cryogenic fluid, for example liquid nitrogen, injectionnozzles are employed on each side of the device.

Shown symbolically in the figure by the reference 5 are the cryogenicliquid injection nozzles connected to the wall of the mixer, the nozzlesthemselves being supplied via hoses 6, by a delivery and supply rail 7,advantageously positioned, as is the case shown in this FIG. 1, abovethe injection nozzles.

In order not to clutter up the figure unnecessarily, the axes of therotor shafts of the mixer are represented by simple crosses with thereference 4, one axis per trough of the mixer as shown in FIG. 1.

As may be understood on examining this FIG. 1, the position of theinjection nozzles along the wall of each trough (the angle β), and alsothe angle of inclination of each injection nozzle with respect to thehorizontal (the angle α), have in this case advantageous values for thepurpose, on the one hand, of preventing the path of the cryogenic liquidjet from crossing the shafts and rotors of the mixer (avoiding any riskof creating cold spots), while involving a maximum portion of the massof product to be chilled, contained in the mixer, but also, on the otherhand, because of the inclination of the injection nozzle with respect tothe horizontal, when subsequently cleaning the mixer with water, ofpreventing this water from being able to get back into the cryogenicliquid supply line.

Thus, it is considered that an angle β with respect to the vertical ofabout 45° gives good results and that an angle α with respect to thehorizontal of at least 10° is a setting that it is advantageous toadopt.

As indicated above, FIG. 3 provides a partial view of the top part of amixing chamber (cover), the top part of which is here provided with aforced convection system formed by two fans having five blades inclinedat 45°. The method represented here is of course only one exemplaryembodiment, many other configurations (numbers of fans, numbers ofblades per fan, inclination, etc.) may be envisaged without departingfrom the scope of the present invention.

The convection system represented in FIG. 3 is that which was used forthe practical and comparative examples (with and without highconvection) related below.

FIG. 5 provides an example of a twin-tube injector that makes itpossible to carry out the mixing at the injection point, and thereforeto achieve an exchange of refrigeration between the two gases at thesame point of injection.

As is preferred according to the invention, the liquid CO₂ passesthrough the outer tube, which promotes the chilling of the CO₂ by thenitrogen, limits heat gains and makes it possible easily to generateVenturi effects on the nitrogen.

As will be clearly apparent to a person skilled in the art, such aninjector will be connected to the device in question, for example ablender with low injection, preferably by quick-connection means,especially for cleanability reasons well known to a person skilled inthe art.

FIG. 6 itself illustrates an example of a three-tube injector that makesit possible to carry out the mixing at the injection point. The methodillustrated here uses the fluids in the following manner:

-   -   liquid nitrogen passes through the inner tube;    -   liquid CO₂ passes into the annular space between the first tube        and the second tube which is concentric thereto;    -   liquid nitrogen passes into the annular space between the third,        outermost tube and the second tube which is concentric thereto.

Here too it will have been understood that this arrangement promotescontact between the two fluids that aims to cool the liquid CO₂.

As already mentioned above, the twin-tube or three-tube injectors inaccordance with the invention, making it possible to carry out themixing at the injection point on the blender, such as those illustratedwithin the context of FIGS. 5 and 6, may be supplied and connected tothe device in question by very simple injector-supply andquick-connection means, but it could also be envisaged to use morecomplex injector feed valves, such that enable the automated control ofthe distribution of the fluids between the various channels of theinjector (for example a valve driven by a pneumatic actuator).

Explained in detail in what follows are the conditions of practicalimplementation examples of the invention and comparative examples, inthe case of a blender for chilling masses of meats:

-   -   use of a Hobart brand blender, having a single trough (as shown        schematically in FIG. 2, and when convection is present it is in        accordance with the appended FIG. 3);    -   low injection system: use of two injectors on the same side of        the trough, and two solenoid valves that are driven at the same        time (it will have been clearly understood that other        configurations of injections, number, on each side, on the same        side, etc. can be envisaged and will be chosen as a function of        the operating conditions, and especially of the type of blender,        of the size of the blender, etc.).

According to the invention, use may be made of very simple, commerciallyavailable injectors such as simple orifices, or else of more elaborateinjectors such as those that the applicant has developed as described indocuments EP 744 578 and EP 2 041 026.

-   -   cryogenic fluid sources used:        -   use of a reservoir of liquid CO₂ stored at 15 bar and −20°            C., which reservoir is placed on a balance in order to            evaluate the consumption;        -   use of a reserve of liquid nitrogen at a pressure of 3.6            bar, which reservoir is here too placed on a balance in            order to evaluate the consumption;    -   the product treated was a boneless manufacturing bulk pack of        fresh bovine meat containing 20% fat that has undergone a first        coarse grinding (freezing point −1° C., water content 62%,        specific heat above the freezing point 0.85 kcal/kg, specific        heat below the freezing point 0.36 kcal/kg, latent heat 55        kcal/kg);    -   the initial temperature of the pre-ground meat is within the        range extending from 3.5 to 4° C., the reference temperature        after grinding is substantially-1° C.    -   the average time for one mixing cycle is customarily, on this        industrial site, 12 minutes, with low CO₂ injection.    -   Protocol followed:        -   Measurement of the temperature of the fresh boneless            manufacturing bulk pack incorporated into the blender.        -   Injection of the cryogenic liquids continuously, with or            without the presence of forced convection (63 Hertz), being            based on two coupled and monitored factors:            -   a temperature of the meat in the grinder of −0.6° C. to                −1° C.;            -   a motor intensity of 6 amperes.        -   At the same time:            -   The intensity of the blender is revealed owing to a                hook-on ammeter.            -   The temperatures inside the blender, on the outside of                the blender (level with the opening/closure of the                cover) and the temperature of the extraction gases are                recorded.        -   Measurement of the temperature of the boneless manufacturing            bulk pack after grinding.        -   Inspection of the visual appearance and the texture of the            ground manufacturing bulk pack and also of the steak formed            downstream by a production manager, inspections complemented            by a bacteriological analysis of the product formed.        -   The tests are carried out on batches of 150 kg of meat            (depending on the test in question, the test is repeated 3            to 4 times to ensure a good reproducibility and            representativeness of the results observed).        -   In all cases, the tests use injectors of simple type (simple            lines connected to the mixer).            -   Tests No. 1—comparative: injection of liquid nitrogen                alone, without use of additional convection.            -   Tests No. 2—according to the invention: tests without                use of convection, injection of the two cryogenic                liquids separately in accordance with FIG. 2 (two                injectors on the same side of the mixer).            -   Tests No. 3: same tests as No. 2 but here with                additional use of high convection.

In other words, these tests are characterized by a constant of theintensity parameters at the end of treatment (6 A), of the freezing hold(the intensity at the end of treatment still being the same), of theboneless manufacturing bulk pack used and pre-ground, of the weight ofbatch treated, of the setpoint temperature after desired grinding(substantially −1° C.).

The results of the tests are assembled in the table presented in FIG. 4below, which results make it possible to draw the following conclusions:

-   -   by the implementation of the invention, a reduction in treatment        time is clearly observed, which reduction is even higher when        additional high convection is used;    -   a decrease in the consumption of fluid used is also clearly        observed. This decrease in consumption must be connected to the        sub-cooling of the CO₂ carried out owing to the invention, but        also to the limitation of the calefaction phenomenon, thus        improving the heat transfer;    -   the presence of forced convection makes it possible to further        improve each of these performances (treatment time and        consumption);    -   in other words, the treatment lasts for less time, the motors        are therefore used for less time, which enables the input of        fewer calories originating from outside and a lower mechanical        energy of the motors transferred to the system;    -   the amount of refrigeration transferred to the mass with respect        to the customary treatment conditions of the site is the same,        the transfer has therefore been improved, whether this is with        the presence of convection or without the presence of high        forced convection. More specifically, the optimization of the        process must be connected to a synergy of cumulated effects.

The use of two fluids makes it possible to operate with a sub-cooledCO₂, injected within the mass of material, while creating a cold gasoverhead, capable of resolidifying the gaseous CO₂ escaping toward thetop of the chamber.

Moreover, it is understood, when a high forced convection is used, thatthis only reinforces these effects.

With regard to the role of this forced convection, these resultsunambiguously demonstrate the fact that this forced convectionintroduced into the top part of the chamber has an unmistakable andpositive effect on the overall transfer of refrigeration carried out onthe mass treated.

This might appear paradoxical considering the compact mass treated, orelse considering the time available during the treatment, which timemight appear to be too short.

It is possible to attempt to provide the following explanation: by wayof comparison, in a conventional cryogenic tunnel, a fan of 0.20 kW/m²is installed in order to achieve a convection of 80 W/m/K. According tothe present invention, and to its credit, it is typically possible toinstall 7.5 kW over a single m², in order to have a convection that canbe estimated at around 200 W/m²/K, which is considerable.

It is possible to consider that the conditions of the invention thenpartly approach impingement type convections.

1-7. (canceled)
 8. A process for chilling products in a chilling device,the device used being a chamber of a mixer, blender, or grindercontaining a mass of product to be chilled, comprising injecting liquidnitrogen and liquid CO₂ within the mass of product in a bottom part ofthe chamber, the injection of liquid nitrogen and liquid CO₂ beingperformed separately at at least two injection points of the device. 9.The chilling process of claim 8, wherein the device comprises a forcedconvection system in a top part of the chamber thereby allowing arecycle and use of a refrigerating power of cold gases that result fromvaporization of the liquid nitrogen and liquid CO₂.
 10. The chillingprocess of claim 8, further comprising the steps of: discontinuing theinjection of the liquid nitrogen and liquid CO₂ for a given stop time;and measuring a temperature in the mass of product to be treated,wherein: if the measured temperature is substantially equal to saidsetpoint temperature the device is changed to shutdown mode and theinjection of liquid nitrogen and liquid CO₂ is completed; and if themeasured temperature is greater than the setpoint temperature, thecontinuous injection of liquid nitrogen and liquid CO₂ is resumed untilthe setpoint temperature is obtained.
 11. The chilling process of claim8, wherein the device is provided with an overflow gas extractor havinga break either in between the device and the extractor or in betweenopposite ends of the device, such that: due to the presence of thebreak, liquids condensing in the extractor are inhibited from beingreturned to the device; and due to the presence of the break, adischarge velocity of cold gases resulting from vaporization of theinjected liquid nitrogen and liquid CO₂ is limited to a value below avelocity that would otherwise allow any solid CO₂ present in the chamberfrom escaping the chamber with the cold gases.
 12. The chilling processof claim 8, wherein the products are food products.